1. Field of the Invention
This invention relates to a water-based, thermosetting polyester coating composition. More particularly, it relates to a polyester coating composition useful as an intermediate coating to be applied on an electrodeposition coating layer on a metallic substrate such as automobile bodies.
2. Background Art
Recently, from the standpoint of saving natural resources and energy and for environmental reasons, much interest has been placed on water-based coating compositions. It is common practice in automobile finish, for example, to apply an electrodeposition coating layer, an intermediate layer of primer or anti-chipping coating and a solid- or metallic color coating layer, successively. If a water-based paint is used as the intermediate coating, not only is it possible to avoid the use of organic solvents, but also the intermediate coating can be applied on the electrodeposition coating layer wet-on-wet and then baked the two layers simultaneously to reduce the number of steps.
Water-soluble or water-dispersible polyester resins including alkyd resins have been conventionally used for such intermediate coatings as the vehicle resin thereof. Known polyester resins of this kind have a problem that the solution viscosity thereof increases as the degree of neutralization with a base increases. Therefore, the solids content of paint compositions should be relatively low in order to achieve a viscosity level thereof suitable for application. This results in poor workability in terms of sagging and so on. Satisfactory workability could be achieved by lowering the degree of neutralization and thus the solution viscosity of the aqueous varnish of the polyester resin. This approach will not be successful because the storage stability of the resulting coating compositions would be significantly impaired. Another approach to formulate a high solids, storage-stable compositions yet retaining a viscosity level suitable for application is the use of a low molecular weight resin, particularly resins having a low weight average molecular weight. The use of low molecular weight resins should fail to impart coating films with a sufficient level of strength and other film performance although the workability and storage stability requirements could be met. Accordingly, what is required for such intermediate coating compositions is that they can be applied to a satisfactory film thickness without developing pinholes, craters, sagging and other film defects while, at the same time, achieving high film performance.
In an attempt to meet these requirements, JP-A-07/252449 corresponding to U.S. Pat. No. 5,597,861 assigned to the assignee of this application discloses a water-based coating composition comprising a polyester resin containing as a polyol component a polyalkadienediol or a hydrogenated product thereof.
Polyester resins to be used for water-based coating compositions must have a number of acid groups corresponding to an acid number of at least 10 mg KOH/g in order that the resin can be dissolved or dispersed in an aqueous medium containing a neutralizing base. One conventional method for introducing such acid groups to the resin molecule is the use of a polybasic carboxylic acid having 3 or more acid groups or an acid anhydride thereof as a portion of polycarboxylic acid component in the synthesis of a polyester chain having a number of pendant free acid groups originating from unreacted carboxylic group or groups in excess of two possessed by the polybasic carboxylic acid. This method increases not only the acid number but also to hydroxyl number of the resin and requires complicated procedure and conditions in the synthesis. Another known method couples a polycarboxyl acid residue having at least one free carboxyl group to a polyester chain through an ester linkage by reacting the corresponding polybasic carboxylic acid anhydride with terminal and/or pendant hydroxyl groups possessed by the starting polyester. This method is used in JP-A-07/252449 and U.S. Pat. No. 5,597,861 cited supra. The polybasic carboxylic acid residues coupled in this manner are not incorporated within the polyester chain backbone and thus they are susceptible to detaching from the polyester chain through hydrolysis of the ester linkage. As a result, the remaining polyester chain decreases in its hydrophilicity and exhibits decreased storge stability represented by increased viscosity or phase separation of coating compositions. The susceptibility to hydrolysis of the resin also decreases the water resistance of coating films thereof.
Therefore, it is an object of the present invention to provide a polyester resin for use in the production of water-based coating compositions particularly intermediate coating compositions to be used in finishing automobile bodies which have not only excellent workability and film performance required for such use but also high resistance to hydrolysis.